Field of the Invention
The invention relates to a measuring and diagnostic device for an ignition system of an internal combustion engine.
German Patent DE 40 39 356 C1 discloses a circuit configuration for determining the discharge voltage of an ignition system of an internal combustion engine. The circuit configuration evaluates the spark plug discharge voltage, which is transformed to the primary side of an ignition coil, through the use of a voltage converter connected as an emitter follower.
German Patent DE 27 59 155 C2 discloses a circuit configuration for detecting the spark burning duration in ignition devices for internal combustion engines. Using an integrator device and a downstream-connected trigger device, the primary-side ignition voltage profile of a transistor-controlled ignition coil is converted with a predetermined response voltage into a square-wave pulse whose duration corresponds to the spark burning duration.
U.S. Pat. No. 4,331,922 discloses a diagnostic device for an ignition system of an internal combustion engine for monitoring the battery voltage, the ignition switch and the spark head voltage. The diagnostic device has a plurality of comparators for comparing the primary voltage with different reference voltages, the comparator outputs being connected via peak value storage circuits to a logic unit which controls devices for indicating faulty conditions.
Since the primary-side ignition voltage across the ignition switch has relatively high voltage levels being above the input voltage range of standard operational amplifiers, particularly complicated clamping circuits are necessary. In addition, the task is not accomplished by the comparators alone, rather a plurality of peak value store circuits are necessary, and also an evaluation logic unit.
In the course of the development of ignition systems in internal combustion engines, it has been shown that it is necessary to protect the ignition switch (power output stage transistor) against destruction if, by way of example, the primary winding of the ignition coil has a short circuit to the positive terminal of the battery voltage source. In addition, for diagnostic purposes, it is helpful to detect the spark burning duration and take suitable measures if the duration lies outside a predetermined desired value range.
A characteristic evaluation signal from which the two phenomena mentioned above can be measured and evaluated is the primary-side profile of the ignition voltage.
With the power output stage switched on, the primary-side ignition voltage has a low level of the order of magnitude of approximately 2V and, after the current flow through the primary winding is switched off at the ignition instant, momentarily rises to approximately 300 to 400V (spark head) until a flashover occurs at the spark plugs on the secondary side. The primary-side ignition voltage then levels out at a value above the battery voltage Vbat which depends on the battery voltage, the burning voltage of the spark gap and on the transformation ratio of the ignition coil. As soon as the energy stored in the ignition coil has then dissipated, the primary-side ignition voltage swings back to a value corresponding to the battery voltage Vbat.
The primary-side ignition voltage Upr thus has relatively high voltage levels which lie above the input voltage range of integrated circuits and necessitate special clamping circuits.